Basic Principles and Techniques

Gorter is given credit for the origin of the concept of nuclear magnetic resonance (NMR) in 1936. y Work was continued by Purcell and Block in the mid 1940s with their Nobel Prize winning discovery of the magnetic resonance phenomenon. Experimentation was limited to very small quantities of materials that were contained within a vacuum chamber. In the late 1960s, Jackson reported NMR experiments on animal tissues. Damadian, in 1972, was the first to describe whole-body NMR for medical diagnosis. Shortly after, in 1973, Lauterbur theorized using magnetic field gradients to create a position-dependent NMR signal. The first MR images of detailed human anatomy were produced by Aberdeen in 1976. The widespread clinical use of MRI began approximately in 1980 and continues to grow at an exponential pace.

NMR is the science that forms the basis for MRI. Abundant water molecules within the human body contain protons that act as microscopic magnets. When the human body is placed into a static main magnetic field, approximately 50 percent of the magnets align parallel to this main magnetic field. The remainder align in an antiparallel fashion. These magnets cancel each other out, although approximately one in a million bar magnets is not canceled out and creates the basis of a tiny magnetic net vector. These microscopic magnets form the basis of the intrinsic signal that is used to generate clinical images. These net magnetic vectors align with the axis of the main magnet and rotate at a specific frequency known as the Larmor frequency. A second type of magnetic field known as gradient fields is used to augment the main magnetic field and allow for anatomical localization of the specific spinning protons. Finally, there is a third type of magnetic field known as a resonance frequency field referred to as a radiofrequency (RF) pulse. This RF pulse is a very low amplitude that oscillates near the Larmor frequency that is 63.87 MHz for a 1.5-Tesla (T) MR image. When the RF pulse is applied, the spinning protons become excited and flip their orientation to a predetermined flip angle, which is usually 90 degrees for a spin-echo technique. When the RF field is turned off, these excited spinning protons convert to a relaxed state, releasing energy. This energy can be measured with head, body, or surface coils and forms the basis for image formation.

The intrinsic high tissue contrast medium for MRI is one of the major strengths of this modality. Whereas CT uses differences in x-ray attenuation coefficients of two adjacent tissues, the MR signal is intrinsic and is generated by changing the proton spins in response to an external RF pulse. The different macromolecular characteristics of the intercellular and extracellular protons within the water molecules determines the intrinsic MR signal that is displayed for diagnostic interpretation. The macromolecular spin magnetization returns to its equilibrium state through a process defined by two relatively independent relaxation times, referred to as T1 and T2.

After a 90-degree RF pulse the proton magnetization net vector rotates from the axis of the main magnet (z axis, longitudinal magnetization) to the transverse plane (x and y axes, transverse magnetization). When the RF pulse ends, the longitudinal magnetization recovers toward 100 percent in an exponential manner. The time it takes for the proton to recover 63 percent of its longitudinal magnetization is referred to as T1 or spin-lattice relaxation. In a similar fashion, when the RF pulse ends, the transverse magnetization is 100 percent and decays toward zero. The time it takes the proton to lose 63 percent of its transverse magnetization is termed T2 or spin-spin relaxation time. The energy signal formed during this decay process is referred to as a free induction decay (FID). This signal is intrinsically weak. It is enhanced by the addition of a second 180-degree RF pulse, which follows the first 90-degree RF pulse. This second RF pulse refocuses the protons and creates a spin-echo signal. The time from the 90-degree pulse to the spin-echo signal is termed the TE. This process is repeated, and the repetition time between the 90-degree pulses is called TR.

Images can be created that have relative T1 weighting (T1WI) or T2 weighting (T2WI) by varying the TR and TE. A T1WI has a TR less than 1000 msec and a TE less than 50 msec. A T2WI has a TR greater than 2000 msec and a TE greater than 60 msec. An intermediate so-called proton density weighing has a TR greater than 2000 msec and a TE less than 40 msec.

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